I have always wondered if there would be any merit to using a portable floor standing air conditioner on an as-needed basis. Could be stored when not needed and should be relatively simple to fabricate a window vent to allow the exhaust hose to be used when needed. Just a thought, but would be a heck of a lot cheaper than a roof mounted unit. For instance, just for fun, check this out (less than 10 amps draw):
Portable Air Conditioner
There are many other options available.
Bingo